What is BMI?
BMI Introduction
BMI tells if your weight is good for your height. It uses your weight and height. It shows if you are too thin, normal, or too heavy. The numbers change with age. It helps find health problems. It is not perfect. Our calculator uses age and kilograms. It also gives a Ponderal Index number.
BMI Table for Adults (Age 20+)
The WHO gives BMI levels for adults 20 and older. It works for men and women. Look at the table below.
| Classification | BMI Range (kg/m²) |
|---|---|
| Severe Thinness | < 16 |
| Moderate Thinness | 16 - 17 |
| Mild Thinness | 17 - 18.5 |
| Normal | 18.5 - 25 |
| Overweight | 25 - 30 |
| Obese Class I | 30 - 35 |
| Obese Class II | 35 - 40 |
| Obese Class III | > 40 |
Our chart shows these levels. Lines mark smaller groups.
BMI Table for Children and Teens (Age 2-20)
The CDC sets BMI levels for kids and teens 2 to 20. It uses age groups. Check the table below.
| Category | Percentile Range |
|---|---|
| Underweight | < 5% |
| Healthy Weight | 5% - 85% |
| At Risk of Overweight | 85% - 95% |
| Overweight | > 95% |
Put your age in the calculator to see your group.
Risks of Being Overweight
Being too heavy can hurt your health. The CDC lists these problems.
- High blood pressure
- Bad cholesterol
- Diabetes
- Heart disease
- Stroke
- Gallbladder issues
- Joint pain
- Sleep problems
- Some cancers
- Low life quality
- Mental health issues
- Body pain
- More death risk
Keep BMI below 25. Ask a doctor for help.
Risks of Being Underweight
Being too thin can cause problems. Here are the risks.
- Not enough food
- Weak bones
- Weak body defense
- Growth issues in kids
- Women’s health problems
- Surgery risks
- More death risk
See a doctor if you are too thin.
Limitations of BMI
BMI helps but is not perfect. It checks weight, not fat. It misses these.
- Muscle and bone
- Age
- Sex
- Ethnicity
- Kid growth
Use other checks with BMI.
BMI Formula
BMI uses weight and height. Here’s how. Example: 5’10”, 160 pounds.
Metric: BMI = weight (kg) / height² (m) = 72.57 / 1.778² = 23.0
US: BMI = 703 × weight (lbs) / height² (in) = 703 × 160 / 70² = 23.0
BMI Prime
BMI Prime checks your BMI against 25. It shows if you are okay.
BMI Prime = BMI / 25
| Classification | BMI (kg/m²) | BMI Prime |
|---|---|---|
| Severe Thinness | < 16 | < 0.64 |
| Moderate Thinness | 16 - 17 | 0.64 - 0.68 |
| Mild Thinness | 17 - 18.5 | 0.68 - 0.74 |
| Normal | 18.5 - 25 | 0.74 - 1 |
| Overweight | 25 - 30 | 1 - 1.2 |
| Obese Class I | 30 - 35 | 1.2 - 1.4 |
| Obese Class II | 35 - 40 | 1.4 - 1.6 |
| Obese Class III | > 40 | > 1.6 |
Below 0.74 is too thin. 0.74 to 1 is good. Over 1 is heavy. Over 1.2 is obese.
Ponderal Index (PI)
PI is like BMI but uses height differently. It helps with tall or short people. Here’s how. Same example: 5’10”, 160 pounds.
Metric: PI = weight (kg) / height³ (m) = 72.57 / 1.778³ = 12.9
US: PI = height (in) / ∛weight (lbs) = 70 / ∛160 = 12.9
PI has limits but fits height better.